In our work, we have studied a crucial fibril forming domain termed the repeat domain (RPT, residues 315-444) derived from the human functional amyloid, Pmel17, to gain insights into what may differentiate functional from pathological amyloid. Pmel17 is a transmembrane precursor protein that is proteolytically processed to form intralumenal fibrils in melanosomes upon which melanin is deposited. Pmel17 is highly regulated in vivo, undergoing a series of post-translational and proteolytic modifications whereby the timing and sequence of these events permit amyloid formation. RPT is essential for the amyloid structures observed in melanosomes. Fibrils are formed during the early stages of melanosome development and once formed are responsible for the deposition of the pigment melanin. Since melanin precursors are cytotoxic, sequestering their synthesis on fibrils prevents potential detriment to the organelle. During this review period, we have extended our biophysical studies of RPT to (1) elucidate the mechanism of how lysophospholipid modulate RPT aggregation and (2) investigate the aggregation propensity of a shorter RPT (sRPT) domain that arises as a result of alternative processing. Previously, we showed that RPT fibrillation is influenced by two lysolipids, the anionic lysophosphatidylglycerol (LPG) and zwitterionic lysophosphatidylcholine (LPC), both of which are enriched in melanosomes. In collaboration with Otzen and coworkers, we investigated the interaction of RPT with both LPG and LPC using small angle X-ray scattering (SAXS), isothermal titration calorimetry (ITC), electron microscopy, fluorescence and circular dichroism spectroscopy. As shown before, both lipids promote fibrillation, however through this work, it is determined that they act differently. Binding to LPG only induces alpha-helical structure well above the cmc, while LPC has no measurable effect on the protein structure. Each RPT binds > 40 LPG molecules, but only weak interactions are seen with LPC. Above its critical micelle concentration (cmc), LPG and RPT form connected micelles, where RPT binds to the surface as beads on a string with core-shell structures. Interestingly, at low micelle concentrations of LPG, aggregation is strongly stimulated, whereas at higher LPG concentrations (10 mM), which one RPT binds per micelle, amyloid formation is inhibited. While ITC and SAXS reveal some interactions between the zwitterionic lipid LPC and RPT below the cmc, no interaction can be detected at above the cmc. Nevertheless, LPC only promotes aggregation above the cmc and this process is not inhibited by high LPC concentrations, suggesting that monomers and micelles cooperate to influence amyloid formation. RPT fibrillates under strict acidic melanosomal pH. Alternative splicing results in a shortened repeat domain (sRPT), which also forms amyloid fibrils. To provide an in-depth mechanistic understanding of the process, we explored the effects of pH and protein concentration on sRPT aggregation by monitoring the intrinsic fluorescence of the sole tryptophan at position 381. W381 emission properties revealed changes of local environment polarity for sRPT fibrils formed at different pH. Using intrinsic tryptophan fluorescence, we show that sRPT aggregation is highly pH-dependent, with fibril formation occurring most rapidly at pH 4>5>6. To identify structural features of the fibrils, we used transmission electron microscopy and Raman spectroscopy, the latter of which revealed variations in the amide-III band. Interestingly, we find that sRPT aggregation at pH 5 displays a unique, concentration-dependent bi-phasic kinetics profile. We attribute the two phases to fibril formation and maturation, respectively. We believe this mechanism of sRPT fibril formation and maturation parallels melanosomal conditions, leading us to propose a potential role for sRPT in vivo.